1. Field of the Disclosure
The present disclosure relates generally to an image forming apparatus and, more particularly, to a system and method for effectively controlling the shape of the electric field in the transfer nip of the image forming apparatus so as to avoid Paschen breakdown and over-transfer of toner in the transfer nip.
2. Description of the Related Art
This present disclosure concerns the transfer process for electrophotographic printers. It applies to both two step toner transfer and direct-to-paper imaging systems. Specifically it applies to the transfer process, whereby toner is moved from a donating medium, such as a transfer belt, to an accepting medium, such as a sheet of paper.
Transfer is a core process in an electrophotographic printing process. The process starts when a photosensitive roll, such as a photoconductor, is charged and then selectively discharged to create a charge image. The charge image is developed by a developer roll covered with charged toner of uniform thickness. This developed image then travels to what is referred to as “first transfer” in the case of a two-step transfer system, or the only transfer process in the case of direct-to-paper systems.
In either system, the toner enters a transfer nip area between a photoconductor roll and a transfer roll. The media to which the developed toner image is to be transferred, either a transfer belt for a two-step transfer system or a transport belt supporting paper for a direct-to-paper system, is positioned between these two rolls. Time, pressure and electric fields all influence the quality of the transfer process. A voltage is applied to the transfer roll to create a field to pull charged toner off the photoconductor onto the desired medium.
In a two-step transfer system, the transfer belt, now carrying the charged toner, travels to a second transfer nip, similar in some ways to the first transfer nip. The toner is again brought into contact with the toner receiving medium in the second transfer nip formed by a number of rolls. Typically a conductive backup roll and a resistive transfer roll together form the two primary sides of the second transfer nip. As with the first transfer, time, pressure and applied fields play significant roles in ensuring high efficiency transfer.
Some existing imaging systems utilize a three-roll configuration for the second transfer nip. Specifically, a transfer roll is disposed along the outer surface of the transfer belt and two rolls are disposed along the inner surface of the transfer belt, with each of the two rolls serving to form the transfer nip with the transfer roll. A description of a three-roll transfer nip is found in U.S. Pat. No. 8,588,667, assigned to the assignee of the present patent application, which is incorporated by reference herein in its entirety.
A three-roll second transfer nip may be wide enough to allow sufficient dwell time for transferring toner using reasonable transfer voltages and roll resistivities. However, the dwell time allows the exit nip region of the second transfer nip to continue to build an electric field until the field reaches the point of Paschen breakdown in the media and/or over-transfer of toner occurs. The result is a loss of operating space for second transfer in some environments, such as class-B environments.
Based upon the foregoing, there is a need for an improved transfer nip in an electrophotographic imaging system.